The required probability of failure is a number representing the probability that an instrumented function will fail in a dangerous scenario. This value is represented as a probability value (e.g., .001), and is an indicator of whether the instrumented function is meeting its requirements to mitigate risk. The required probability of failure value is based on the following values stored in the Instrumented Function:
The following sections describe how the required probability of failure is calculated based on the SIL value, assessment method, and demand rate.
In an Instrumented Function, when the value in the SIL Mode field is Low Demand, the GE Digital APM system assumes that the demand rate of the instrumented function is less frequent than once per year. In these cases, the failure rate is measured by the average probability of failure on demand (PFD Avg).
| SIL Value | Required Probability of Failure (PFD Avg) |
|---|---|
| 4 | ≥ 10-5 to < 10-4 |
| 3 | ≥ 10-4 to < 10-3 |
| 2 | ≥ 10-3 to < 10-2 |
| 1 | ≥ 10-2 to < 10-1 |
In an Instrumented Function, when the value in the SIL Mode field is High Demand or Continuous, the GE Digital APM system assumes that the demand rate of the instrumented function is more frequent than once per year. In this case, the failure rate is measured by the average probability of dangerous failures per hour (PFH). The required probability of failure is defined by an industry standard that is associated with the SIL.
The required probability of failure is defined by the industry standard that is associated with each SIL. The following table contains the standard PFD Avg values for each SIL. The following table contains the standard PFH values for each SIL.
| SIL Value | Required Probability of Failure (PFH) |
|---|---|
| 4 | ≥ 10-9 to < 10-8 |
| 3 | ≥ 10-8 to < 10-7 |
| 2 | ≥ 10-7 to < 10-6 |
| 1 | ≥ 10-6 to < 10-5 |
If you use a Risk Matrix (internal) in SIL assessment, the value in the Required Probability of Failure field is disabled and populated with the lowest (i.e., the most conservative) value as the targeted probability. For example, if the SIL value is 2, the Required Probability of Failure field will be populated automatically with 1E-07 (i.e., 10-7) in case of a Low Demand SIL mode.
If you use an Layer of Protection Analysis (internal) in SIL assessment, the value in the Required Probability of Failure field is disabled and populated with the difference between the values in the Mitigated Consequence Frequency field and the Required Mitigated Consequence Frequency field in the LOPA.
If you use any external SIL Assessment method, the Required Probability of Failure field will be enabled, and you can enter the probability value manually. In this case:
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